The present invention relates to communications in wireless networks. More specifically, but not exclusively, the present invention relates to signaling techniques for adaptively modulating communications in a high throughput wireless network.
Most communications networks are designed to convey multiple communications simultaneously over each individual communication path, for example, a radio frequency (RF) channel, using some type of modulation. In recent years, an increasing demand has arisen for more efficient and reliable digital data transfers which assure correct data transmissions at as high a data rate as possible.
Orthogonal frequency division multiplexing (OFDM) is an increasingly attractive modulation technique for high-bandwidth wireless applications since it dramatically simplifies equalization of intersymbol interference (ISI) channels. Using link adaptation (LA), it is possible to improve throughput and/or efficiency in wireless OFDM systems by adjusting transmission parameters, such as subcarrier modulation orders, power allocation and/or code rate, to best fit the current channel state.
Ideally, link adaptation would adapt at every time instant in frequency to the instantaneous channel realizations. Unfortunately, limitations in feedback bandwidth and variation of the channel due to Doppler spread make ideal link adaptation difficult to realize.
One of the difficulties encountered in LA for wireless networks, for example, high throughput (HT) wireless local area networks (WLANs) with adaptive OFDM, is the useful duration of channel adaptation information. Since propagation channels can change rapidly due to Doppler and other effects, the useful duration of channel adaptation information may be dependent on the coherence time of the channel. Coherence time is the time domain dual of Doppler spread (i.e., Doppler spread and coherence time are inversely proportional to one another) and is used to characterize the time varying nature of the frequency dispersivenes of a channel in the time domain.
Coherence time is a statistical measure of the time duration over which the channel impulse response is essentially invariant, and quantifies the similarity of the channel response at different times. In other words, it is the time duration over which two received signals have a strong potential for amplitude correlation. Network environments with long channel coherence times may not need as frequent channel adaptations as network environments with shorter channel coherence times. Thus the channel adaptation information for longer channel coherence times may be exchanged on a proportionately less frequent basis. Conversely, networks in highly dynamic environments may need to exchange adaptation information more often in order to maximize the efficiency of the channel adaptations. A method, system and/or technique for efficient link adaptation between communicating devices is needed.